Randyll writes: During its Saturn flyby in March the Cassini space probe detected an oxygen-rich atmosphere on Rhea, Saturn's second-largest moon. While 100 times thinner than the atmospheres of Europa or Ganymede, the atmosphere contains a surprising amount of carbon dioxide. There is an explanation for the oxygen, while the origin of the carbon dioxide is a mystery. A few of the possible explanations are that Rhea has carbon-rich organic molecules or that the gas is seeping from Rhea's interior, however researchers have been unable to determine the exact source for the gas.